Air Force Gen. Philip Breedlove, Supreme Allied Commander Europe and chief of the U.S. European Command, said new missions to keep U.S. ground troops and aircraft in Eastern Europe, including Poland, the Baltics and Romania, are putting a strain on his current force of about 67,000 troops.

"We may need to add additional rotational forces to cover the sustained and persistent presence that we are now envisioning," Breedlove told reporters during a visit to Washington on June 30.

He pointed to the example of rotational forces arriving in October, when soldiers from the Texas-based 1st Cavalry Division and assigned to the NATO Response Force will deploy to EUCOM for six months to support expanded operations.